In such telecommunication systems it is known to use frequency-division multiplexing (FDM) for accommodating different channels, referred to hereinafter as trunk lines, on a common inter-office transmission link. The establishment of a connection is relatively time-consuming; generally, such systems do not permit the interruption of the ongoing conversation by either the calling or the called party for switching purposes, e.g. to set up a conference call, without the release of a previously engaged trunk line and the consequent loss of time in seizing another one.